MadSci Network: Physics |
In the classical model of electric conduction, a conductor (ie. metal bar, wire, etc.) is pictured as a three dimensional array of atoms or ions and the electrons are free to move about the conductor. In the absence of an electric field the elctrons move about in the same manner as gas molecules move about in a container. The free electrons collide with ions of the 3-D array and are in thermal equilibrium with them. The speed with which the electrons are "bouncing" around is on the order of 10^7 cm/s. When a potential is electric field is applied, the electron experiences a force and subsequently it is accelerated. The velocity of that electron is proportional to the force and the duration that the force is applied. It is inversely proportional to the mass of the electron. velocity = q * E * t / m where q is the charge, E the field strength, t the duration, and m the mass. So in other words there is no "constant" speed for electricity. The net speed with which electrons travel under some field is called the drift velocity. Here is an example of how to calculate the drift velocity for electrons in a conductor carrying current: example ------- Say you have a piece of 14 gauge copper wire (radius 0.0814 cm) and that wire is carrying 1 A. We can assume one free electron for every copper atom in the wire. The density of free electrons in the wire, n, is n = (6.02 x 10^23 atoms/mol)(8.92 g/cc) / 63.5 g/mol = 8.46 x 10^22 atoms/cc and the drift velocity, v, is v = 1 C/s / (pi * 0.0814 cm * 0.0814 cm)(8.46x10^22 atoms/cc)(1.6x10^-19 C) = 3.55 x 10^-3 cm/s = 0.00255 cm/s Note that this drift velocity is very small when compared to the velocity the electron at thermal equilibrium is at "bouncing" around. [Moderator note: Another way to look at this is from the perspective of an electron. For the wire mentioned above, when the current flows, it is just one electron being pushed into the wire at one end, and another electron popping out the other end, over and over many times a second. The electrons in between shift over with a speed of 0.00255 cm/s. However, the speed of the "push" is close to the speed of light, so that the electron on the far end of the wire pops out very soon after the electron on the other end is pushed in. I think your question brings up the additional question of what exactly electricity is. Is it the electrons, or the movement of electrons?]
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